Effect of forage particle size and feeding frequency on fermentation patterns and sites and extent of digestion in dairy cows fed mixed diets

This trial aimed at studying the effects and possible interactions between forage particle size and feeding frequency on the site and extent of digestion and ruminal fermentations throughout the day in dairy cows fed mixed diets. Two forage particle sizes (chopped, C, or ground and pelleted, G, dehydrated alfalfa) and two feeding frequencies (two (2) or six (6) times daily) were combined in a 2x2! arrangement. Four mid-lactation cows with cannulae in the rumen and proximal duodenum were used in a 4x4 Latin square design. They were fed at 95% of ad-libitum intake with three hours access to the manner after each meal distribution. On a dry-matter (DM) basis, the four diets consisted of 40% dehydrated whole-crop maize, 4% long perennial ryegrass hay, 25% dehydrated alfalfa and 31% concentrate (69% wheat). Dry-matter intake averaged 22.2 kg day(-1). No interaction between forage particle size and feeding frequency was found. Feeding G rather than C diets reduced chewing time (22.4 vs. 26.2 min kg(-1) DM), total and ruminal digestibilities of organic matter (OM) and cell-wall constituents (p<0.10), and increased the non-ammonia nitrogen flow into the duodenum per unit of digestible OM intake (p>0.05). Feeding G diets also reduced the molar proportion of acetic acid and increased the molar proportions of propionic and valeric acids. These changes in ruminal digestion were associated with a lower fibrolytic activity and modifications in ruminal environment. The daily means of pH and ammonia concentration were decreased, while the range of pH during the day and the time when pH and ammonia concentration were low (<6.0 and 2.9 mmol l(-1), respectively) were increased. At the same time, the proportion of fluid in the reticular contents and the fractional outflow rate of liquids and small particles were decreased. Increasing feeding frequency did not affect carbohydrate digestion and mean ruminal parameters, although post-feeding variations of pH, osmolality and ammonia concentration were much less pronounced. It is concluded that reduced forage particle size can sharply affect the quantity and the nature of digestive end-products by altering chemical and physical conditions in the rumen. However, the magnitude of this effect is not dependent on the amount of fermentable OM entering the rumen in one meal. (C) 1999 Elsevier Science B.V.